As is well-known in the television art, the raster or deflection pattern produced on a spherical face of a cathode ray tube (CRT) suffers pincushion distortion as the center of deflection of the electron beam deviates from the center of curvature of the screen. With non-spherically faced CRT's, the effect of pincushion distortion is more pronounced. Trapezoidal and other types of distortion are also introduced if the yoke (deflection winding structure) is not accurately positioned on the neck of the CRT. Compounding the problem, is the multi-gun shadow mask type of CRT which requires that the plural beams from the electron guns converge at the phosphor target for color purity. The distortion correction circuits of the prior art add appropriate currents to the deflection yokes for straightening the top/bottom, and left and right sides of the scanned raster. As will be apparent to those skilled in the art, the yoke structure itself may be designed to compensate for a portion of the pincushion error. It is common to incorporate a combination of mechanical correction in the yoke with electrical pincushion correction circuitry.
With the ever-increasing use of CRT's in monitor applications, where data is displayed rather than video or picture information, the constraints imposed upon the degree of pincushion distortion, misconvergence and other forms of distortion have become much more rigid. In monitor applications, for example, it is imperative that the edges of the raster be capable of close control (as to size and linearity) to assure a display field having correct dimensions and straight sides.
The circuits of the prior art develop pincushion correction signals that generate parabolic yoke currents, with controllable phase, amplitude and tilt characteristics, for affecting both the top and the bottom of the raster in substantially the same way. These circuits have been generally satisfactory for use with less stringent television displays, but require excessive setup time (including careful yoke positioning adjustments) to meet the more exacting standards for CRT raster displays in monitors. Further, the problem is exacerbated with color tubes of the flat tension mask variety which have a flat faceplate and an in-line gun structure and wherein yoke construction and positioning may be compromised to enhance beam convergence throughout the raster. It would be extremely beneficial to provide pincushion circuitry having independent control effects at the top and the bottom of the raster.